An autoinjector is an automatic injection device designed to facilitate delivery of a dose of medicament to a patient through a hypodermic needle, the injection usually being administered by the patient themselves. An autoinjector works, for example, by delivering an injection automatically upon actuation by the patient pressing a button, moving a lever or part of a housing etc. This is in contrast to a conventional manual syringe where the patient themselves needs to directly depress a plunger into a barrel containing medicament in order to effect the injection. The terms “autoinjector” and “injection device” are used interchangeably in the following description.
One typical known autoinjector is described in WO00/09186 (Medi-Ject Corporation) for “Needle assisted jet injector” and this document gives a useful summary of other prior art devices.
Another autoinjector is described in our co-pending international patent application, published under number WO 2005/070481. Some of the reference numerals in the present application correspond with the equivalent components in the device described in WO 2005/070481. This device requires that the needle is moved axially so that it can appear beyond the end of the nozzle for the duration of the injection, after which the needle retracts automatically, so that it is never in sight of the user. The device also requires that the plunger is moved axially so that medicament is ejected. The overall complexity of the autoinjector is significantly reduced by both of these requirements being effected by one component, namely an inner housing and the device has the significant advantage that it can be built around a conventional or standard syringe presentation.
The injection device of WO 2005/070481 is designed to be used in conjunction with a standard drug presentation e.g. a syringe comprising a needle, barrel preloaded with medicament and a plunger. The present invention is relevant to any injection device for use in conjunction with a syringe (whether preloaded or not and whether single-use or reusable), not only the device described in WO 2005/070481.
In the known device described in our co-pending patent application no WO 2005/070481, the syringe is supported within the injection device by a barrel or syringe holder 9. The syringe holder is sometimes referred to as a “syringe support means”. The syringe holder 9 comprises an elongate housing which closely surrounds the glass barrel of the syringe. An improved syringe holder is described in our co-pending UK patent application number 0620163.6 filed 12 Oct. 2006. During delivery of an injection, the syringe holder and syringe contained therein are moveable along an axial path, substantially parallel with the longitudinal axis of the autoinjector.
A potential problem arises when the needle cover of an autoinjector is removed, in preparation for delivering an injection. An autoinjector is usually supplied to the patient with the needle of the syringe embedded in a rubber or other elastomeric sheath. The rubber sheath is in turn closely surrounded by a rigid needle cover which protects the needle from damage. Both the rubber sheath and rigid needle cover need to be removed before an injection can be delivered. Actuation of the autoinjector to deliver an injection occurs by actuating the main energy source (usually a spring) of the autoinjector. Prior to that, removal of the rubber sheath and rigid needle cover is usually achieved by providing some kind of gripping means on the interior of the autoinjector's end-cap, so that when the patient pulls the end-cap off the device, the rubber sheath and rigid needle cover are simultaneously removed with the end-cap. In a device such as that described in WO 2005/070481, even when ready to deliver an injection, the unsheathed needle is not exposed to the patient because it is located wholly within the autoinjector's housing.
As the rubber sheath is pulled from the needle, the needle is subjected to a forward axial force which in turn pulls the syringe (to which the needle is attached), moving it slightly axially forward. When the needle comes free of the rubber sheath, the forward axial force is suddenly removed and the needle and syringe can “bounce back” against other internal components of the autoinjector to its original axial position.
It is therefore an object of the present invention to provide an improved autoinjector which seeks to alleviate the above-described problems.